Inter-carrier interference (ICI) is a common problem in mobile communications systems. When receivers are mobile, interference not only results from multi-path environments leading to multiple echoes received in the time domain, but also from frequency shifts that independently affect these echoes and the original signal. Such frequency shifts are known as Doppler frequency shifts, and are directly proportional to the radio channel frequency and the receiver speed.
The Doppler frequency shift fd can be estimated as follows:fd=(v·fC)/C where v is the speed of the receiver, fC is the carrier frequency, and C is the speed of the wave, which can be assumed to be 3×108 m/s. Thus assuming a speed v of 140 km/h, and a central carrier frequency in the region of 800 MHz, this leads to a Doppler frequency shift of approximately 100 Hz. In close-packed multiple carrier communications systems this could lead to inter-carrier interference that prevents signals from being successfully received.
OFDM (orthogonal frequency-division multiplexing) modulation is a frequency-division multiplexing scheme according to which packets of data are modulated on a large number of closely packed orthogonal sub-carriers. Each sub-carrier is modulated by a conventional modulation scheme, such as QAM (quadrature amplitude modulation) or PSK (phase shift keying). Data is divided into multiple parallel streams, one for each sub-carrier, and transmitted in symbols comprising all of the modulated sub-carriers.
Digital video broadcasting standards such as DVB-T (digital video broadcasting of terrestrial television) and DVB-H (digital video broadcasting to handheld devices) propose the use of OFDM modulation, and can be applied to wired or wireless signal transmission channels, and to various applications such as digital television and audio broadcasting, wireless networking and broadband internet.
An advantage of OFDM modulation is its ability to cope with severe channel conditions, such as frequency selective fading. However, due to close-packed carriers, there is a problem in adequately dealing with inter-carrier interference in OFDM systems, particularly when a receiver and/or transmitter is moving at relatively high speeds.